1. Field of the Invention
This invention relates to a method for conditioning a coal seam prior to underground coal gasification wherein the permeability of the coal seam to gases has been substantially increased.
2. Description of the Prior Art
In developing the underground coal gasification (UCG) process wherein a combustion front is moved from an injection well to a production well, it is first necessary to establish a high gas permeability zone between these wells prior to ignition of the coal with an oxygen-containing gas, otherwise combustion cannot be carried out efficiently. To establish the high gas permeability zone, a number of methods have been employed such as fracturing the coal seam with explosives, pneumatic and hydraulic fracturing, directional drilling, and reverse combustion linking. Such methods, however, have inherent limitations. For example, fracturing the coal seam is not very successful because it frequently happens that fracturing takes place beyond the boundaries of the coal seam. Water can then leak into the seam from an overlying aquifer and injected combustion-supporting gas and product can be lost through the overburden. Moreover, fracturing may not be evenly distributed throughout the seam, leading to under-utilization of the coal during gasification. Also, it is necessary to shut down the operation when it is desired to start combustion in a new portion of the seam. This makes for frequent cessation of operation and increase in cost. In the case of the reverse combustion linking process, a combustion-supporting gas such as air is injected at high pressure and low volume in one process well and the coal is ignited in the other process well. The thermal front is then drawn toward the air source leaving a highly permeable char zone. It can be seen, however, that the reverse combustion linking process may fail to work if there are insufficient air flow paths to enable the combustion-supporting gas to pass through the coal seam and establish the link along the bottom of the seam.
As an alternative approach to the problem of increasing the permeability to gas in a coal seam, it appears that a method designed to (1) reduce the water content of the coal, and (2) alter the wettability of the coal, would be highly beneficial. This is because, in the first instance, the removal of water from coal per se results in an increase of permeability for the flow of gases. The relation between the relative permeability of a water-containing coal seam to gas as a function of water content or saturation is shown in FIG. 3 on page 8, in a paper by R. E. Glass entitled, "Applications of Reservoir Simulation to In Situ Coal Gasification," March 1981, Department of Energy Contract No. DE-AC04-76DP00789. These results clearly show that the relative permeability of a coal seam to gas increases as the water saturation decreases.
Another article by Lien et al., "Permeability Characteristics of Coal Seam," Proceedings of the Third Annual Underground Coal Conversion Symposium, Fallen Leaf Lake, California, June 6-9, 1977, pp. 454-465, shows the relation between the relative permeabilities to gas as a function of water saturation for various ranks of coal.
As is evident then, it would be largely desirable to condition a coal seam so as to increase gas phase permeability of a coal seam prior to in-situ gasification so as to increase the efficiency of the UCG process. As indicated, gas permeability rises rapidly as water is removed from a coal seam.
The benefits of a reduced water content will be further enhanced, to a significant degree, if the wettability of the coal is altered. This is because wettability is an important factor in determining the dependence of gas relative permeability on water saturation. Thus, very low rank coals are normally highly water-wet and have a low gas permeability in the intermediate range of water saturations. Higher rank coals (in the range of 90 percent carbon) are significantly less water-wet and have considerably higher gas permeabilities for intermediate water saturations. It follows, thus, that altering the wettability of a very low rank coal in such a way that the coal is less water-wet will greatly increase the gas permeability of the coal.
The increase in gas permeability may therefore be accomplished both directly, as a consequence of the increase in gas relative permeability for a given water saturation, and indirectly, since for a given gas phase driving pressure the water saturation is more effectively decreased as the result of the flow of gas. A further benefit results from the increase in the water phase relative permeability which occurs when the coal becomes somewhat less water-wet. An increase in water phase relative permeability also enhances the removal of water from the coal and thereby contributes to the increase in gas permeability.